South Korea sent two helicopters to battle a wildfire inside the Korean Demilitarized Zone (DMZ), Seoul’s military said Friday, adding it had warned North Korea in advance of its firefighting mission along the heavily armed border area.

South Korea fired warning shots after, Seoul says, North Korean troops crossed their border. The soldiers then went back to the North, Seoul said.

In an unexpected border intrusion, about 10 North Korean soldiers crossed the tense border between North and South Korea, prompting the South to fire warning shots. This marks the first known intrusion by North Korea in nearly a year.